In battery-powered portable terminal apparatuses, liquid crystal display apparatuses have been proposed which reduce power consumption. An example is disclosed in Japanese Laid-Open Patent Publication HEI5-53541 in which the access to a video memory is monitored for a predetermined time to control a driving section of a display apparatus, or in Japanese Laid-Open Patent Publication HEI9-212134 in which updated part of display data is only transferred to a display memory. A conventional moving picture decoding display apparatus that is a liquid crystal display apparatus will be described with reference to FIG. 1.
FIG. 1 is a block diagram illustrating a configuration of a conventional moving picture decoding display apparatus. Generally, moving picture coding/decoding schemes such as ITU-T H.263 and MPEG use image formats such as CIF (Common Intermediate Format) and QCIF (Quarter CIF). Herein, as an example, a case is explained that the image format is of CIF4:2:2 (Valid display area: 352 pixels×288 scanning lines).
Moving picture decoding display apparatus 50 illustrated in FIG. 1 is provided with moving picture decoder 51, video signal converter 52 having delay line section 53 and RGB matrix manipulation section 54, and liquid crystal display module section 55 having video RAM 56, contrast/brightness adjuster 57, driving signal converter 58 and liquid crystal panel 59.
Moving picture decoder 51 outputs video signals in YCbCr format. The video signal in YCbCr format includes brightness signal Y, color-difference signal C on which time-division multiplexed are two color-difference signals (Cb and Cr), vertical sync signal VDN, horizontal sync signal HDN, and clock signal VCK used in transferring video data.
Video signal converter 52 has RGB matrix manipulation section 54 that converts video signals in YCbCr format into LCD_R, LCD_G and LCD_B in RGB format using brightness signal Y and color-difference signal C, and delay line section 53 that adds a delay equivalent to RGB matrix manipulation section 54 to vertical sync signal VDN and horizontal sync signal HDN output from moving picture decoder 51, and outputs vertical sync signal VSYNCN and horizontal sync signal HSYNCN to liquid crystal display module section 55.
Liquid crystal display module section 55 has video RAM 56 that temporarily stores a video signal in RGB format output from video signal converter 52, contrast/brightness adjuster 57 that adjusts a dynamic range and offset amount of the video signal in RGB format read from video RAM 56, driving signal converter 58 that generates a driving signal for driving liquid crystal panel 59 from vertical sync signal VSYNCN and horizontal sync signal HSYNCN output from video signal converter 52 and R′G′B′ signal output from contrast/brightness adjuster 57, and liquid crystal panel 59 that displays moving pictures corresponding to the driving signal.
The operation of moving picture decoding display apparatus 50 with the above configuration will be described with reference to FIGS. 2 to 4. FIG. 2 is a timing diagram of input and output signals in the video signal converter in the conventional moving picture decoding display apparatus, FIG. 3 shows graphs illustrating input/output characteristics to explain the processing in the contrast/brightness adjuster, and FIG. 4 is a diagram illustrating moving pictures to be displayed on the liquid crystal panel.
Moving picture decoder 51 outputs to video signal converter 52, as shown in FIG. 2, as moving-picture video signals, vertical sync signal VDN, horizontal sync signal HDN, brightness signal Y (8 bits) and color-difference signal C (8 bits) with time-division multiplexed two color-difference signals (Cb and Cr), in synchronization with a falling edge of clock signal VCK.
Moving picture decoder 51 inputs brightness signal Y and color-difference signal C to RGB matrix manipulation section 54. In the video signal in CIF4:2:2 format, as shown in FIG. 2, two color-difference components, i.e., Cb and Cr are time-divided. A color-difference component of an odd-numbered pixel number is interpolated by a color difference of an even-numbered pixel number, converted into the signal in RGB format, and output as signal LCD_R, LCD_G and LCD_B. Specifically, the section 54 calculates as described below.
LCD_R=Y+1.402*Cr
LCD_G=Y−0.344*Cb−0.714×Cr
LCD_B=Y+1.772*Cb
Herein, it is assumed that each has 6 bits.
Meanwhile, moving picture decoder 51 inputs vertical sync signal VDN and horizontal sync signal HDN to delay line section 53. Delay line section 53 adds a processing delay of RGB matrix manipulation section 54 to the input signals, and outputs vertical sync signal VSYNCN and horizontal sync signal HSYNCN to liquid crystal display module section 55.
Liquid crystal display module section 55 temporarily stores provided video signals LCD_R, LCD_G and LCD_B in video RAM 56 every rising edge of VCK. Video RAM 56 reads out the signals as R, G and B to output to contrast/brightness adjuster 57.
The input/output characteristics that are functions of contrast/brightness adjuster 57 are expressed by Y=Contrast(X−32)+32+Brightness where an input is X and an output is y, as shown in FIG. 3. These signals are processed for each of R, G and B. When an output range is of 6 bits, since the range is from 0 to 63, the output Y exceeding the range undergoes clipping to be in the range of 0 to 63.
These characteristics can be changed with values of contrast and brightness given from the outside as parameters. The processed output signals R′, G′ and B′ are provided to driving signal converter 58 along with vertical sync signal VSYCN and horizontal sync signal HSYNCN, and the section 58 generates a signal for driving liquid crystal panel 59 to display moving pictures.
It is generally known that liquid crystal panel 59 should drive video signals in about 60 Hz so as to suppress the flicker of displayed image. In other words, image display intervals in FIG. 4 need to be one-sixtieth seconds. In conventional moving picture decoding display apparatus 50 as described above, the video signal is output from moving picture decoder 51 at 60 Hz, and the processing in video signal converter 52 and subsequent sections is executed at 60 Hz consistently.
However, in the conventional apparatus, there are problems as described later. That is, the decoding processing in moving picture decoder 51 is dependent on its performance and handling image size, decoded image quality, coding bit rate, etc., and decoded images are not always different every 60 Hz. In other words, although images are output at a frame rate of 60 fps, some sheets of same image are provided actually. As shown in FIG. 4, image frames with the same contents, for example, three sheets of image A and two sheets of image C, are present. The actual frame rate in portable video telephones is estimated at approximately 15 fps. The processing at 60 Hz is required for reading data from video RAM 56 and thereafter, and the processing at 60 Hz in video signal converter 52 disposed before video RAM 56 results in current dissipation.
Further, contrast/brightness adjuster 57 disposed after video RAM 56 also executes processing at 60 Hz always, resulting in disadvantages in current consumption.